COMP: do not export package on build tree

export(PACKAGE VegaFEM) was resulting in Vega being registered
in a CMake Package Registry :
https://cmake.org/Wiki/CMake/Tutorials/Package_Registry

Because of this, an incorrect version of VEGA could be directly
linked against when doing find_package(VegaFEM).

This should only be done when creating a package for installation:
https://cmake.org/cmake/help/git-master/manual/cmake-packages.7.html#creating-packages